Flue gas process



March 21, 1967 R. H. DUNDAS ETAL 3,310,365

FLUE GAS PROCESS Filed 00's. 30, 1962 D NM S N UO V| v w ET E sm m SV 5G52 H wm w T a. m Dn E T B R O E 2@ und R B 3.68 m w\ m Q N .c

Pr o. i@ ESN: mma NN @N N N NNN/NGN Nm m N NN m I om .Ml .mi m56@ E N, @Nmrm ,N ES?. N252 mmmo United States Patent O M 3,310,365 FLUE GAS PROCESS Robert H. Dundas, Whittier, and Albert E. Gosselin, Los

Angeles, Calif., assignors to Southern California Edison Company, Los Angeles, Calif., a corporation of California Filed Oct. 30, 1962, Ser. No. 234,177 11 Claims. (Cl. 23-2) This invention relates generally to the handling of gases containing sulfur trioxide and to the use and treatment of gases containing sulfur trioxide. This application is a continuation-in-part of our copending application Serial No. 223,151, tiled September 12, 1962, on Air Pollution Control. Y

The present invention is primarily concerned with the operation of oil-tired steam generating equipment, although in its broadest aspects, as will be apparent hereinbelow, the invention is not so limited. Operators and designers of thermal electric generating plants are continually striving for higher'eiciencies, i.e., a higher conversion of the fuel energy into electrical output. The boiler or steam generating equipment used in such plants -constitutes a key factor in the development of improvedv efficiency. Elliciencies of the order of 80% conversion of fuel energy into steam energy was at one time considered excellent, but in recent years, through the use of improved surface absorption and distribution design 'and yalso through the use of improvement for the recovery of waste exhaust heat in the ilue gases, eiiciencies in the neighborhood of 91% have been reached with oilvfired steam generators.

.Overall plant efficiencies have increased to a point where, prior to the present invention, it was commonly believed by those skilled in the art that further appreciable increases, even of the order of one or two percent could only be realized through the costly use of very high pressures and temperatures in the boiler-produced steam. This, notwithstanding the fact that those skilled in the art have long been aware that additional heat is available for recovery from the exhaust or flue gases. Recovery of a portion of the waste heat in the exhaust, through use of air preheaters and the like, wherein the air used for combustion of the fuel is preheated by heat-exchange with the hot exhaust gases has advanced to the point where modern operation of air preheater equipment is carried out under conditions so as to transfer to the air very appreciable percentages of the waste heat in the exhaust. However, the art :has recognized that as the exit gas temperature and/or cold end metal temperature of the air preheater is decreased, the sulfur trioxide-induced corrosive tendencies ofthe exhaust gases are progressively increased, resulting in material replacements and loss of boiler reliability. Accordingly, most modern oil-tired boilers .are operated with exit gas temperatures from the air preheaters not lower than about 290 F., and even here, the design of most plants includes an air heater for the purpose of heating the incoming ambient air to the air preheater and to thus maintain the cold end metal temperature of the air preheater at all times above about 185 F.

Contrary to the above considerations and the heretofore accepted postulate that the corrosive influence of flue gases increase as the llue gas temperature and/or cold end metal temperature is decreased, applicants have 'discovered that a critical cold end metal temperature vexists for gases containing sulfur trioxide, below which temperature the corrosive tendencies of the gas no `longer increase and in fact tend to decrease until an even lower temperature is reached at which point these corrosive tendencies again increase. Applicants have further discovered that this critical temperature, for the ygaseous products of combustion of sulfur-containing fuel oil is in fact consider-ably below the acid `dew point temperature of the gas when the cold end metal temperature is decreased accordingly. Therefore, it is a primary object of the present invention to provide Ia process utilizing these phenomena.

Another object of the present invention is to provide a process for the operation of an oil-fired steam generator of the type including an air preheater, wherein appreciable increases in eiciency of operation are obtained.

Another object of the present invention is to provide a process for the operation of an air preheater system in 4a manner to obtain maximum recovery of heat from the hot gases passing therethrough.

A further object of the present invention is to provide a process wherein the above-mentioned advantages in increased eiiciency of` operation are combined with further processing steps wherein sulfur trioxide contained in the exhaust gases passing through the air preheater is removed.

Still another object of the present invention is to provide an improved process for the control of .air pollution caused by sulfur trioxide in exhaust gases.

Other objects and advantages of the present invention it is believed will be readily apparent from the following detailed description of preferred embodiments thereof when read in connection with the accompanying drawings.

In the drawings:

FIGURE l is a diagrammatic view illustrating a system and apparatus useful in carrying out the process of the present invention.

Brielly, this invention comprehends within its scope the discovery that the corrosiveness of gases containing sulfur trioxide in contact with metals such as steel does not further increase as the gas is cooled below a temperature corresponding to a critical cold end metal temperature, and further that upon additional cooling, below the acid dew point temperature of the gas, a second critical cold end metal temperature is reached at which corrosion is reduced below that obtained at either higher or lower temperatures. These critical temperatures depend upon the concentration of sulfur trioxide in the gas and are inlluenced to some degree by the amount and kind of contaminants in the gas, such as for example ash and dust normally encountered in Hue gases. Generally however, for operation of an air pre-heater wherein ambient air is heated by heat exchange with the sulfurtrioxide containing gaseous products of combustion, the first critical temperature corresponds to a cold end metal temperature (the average of the exit gas temperature from and the inlet air temperature to the air preheater, com- `monly known as CEMT) of about 170 F. (i5 F.),

and the second critical temperature corresponds to a CEMT in the range of about F. to as low as about 120 F. Thus, as the CEMT is brought below about F., the corrosion rate does not increase, and in fact begins to decrease, until an optimum minimum rate of corrosion is reached at a second critical CEMT within the range 15S-120 F. Further reduction in CEMT below the second critical point results in an increase in corrosion. A preferred CEMT range, for operation with exhaust gases from an oil-tired boiler, which generally contain of the order of 20 p.p.m. S03, is 145 130 F.

The use of the low CEMTs in accordance with the present invention produces ideal conditions for removal and recovery, if desired, of the sulfur trioxide present in the exhaust gas. That is, it has been found that operation at the low CEMTs brings about the forma-v aerosol is readily removed from the gas stream in any convenient maner, for example, by operations such as scrubbing, centrifuging, electrostatic precipitation and other precipitation methods.

Further comprehended within the scope of this invention is the discovery that the critical reaction temperature disclosed in said copending application Ser. No. 223,151 for removal of sulfur trioxide from gaseous products of combustion by injection into the gas stream of quantities of a uely divided solid alkaline material i.e. temperatures at or below the acid dew point value of the gas, overlap the critical temperatures referred to above in connection with corrosivity. Accordingly, included within the scope of the present invention is a combined process, wherein the benefits in increased efliciency of operation of a steam generating plant permitted by operation of the air preheater at a CEMT suiciently low to utilize the majority of the waste heat in the exhaust gas, are supplemented by removal of the sulfur trioxide in the exit gas from the air preheater, which exit gas has been cooled in the air preheater to the necessary critical reaction temperature for use of the alkaline material.

In carrying out one embodiment of the present invention, constituting a process for heat exchange between hot gases containing sulfur trioxide and air to be used as for example in the combustion of the fuel producing the hot gas, any type of preheater can be utilized. Preferably, the process lis carried out with an air preheater of the regenerative type such as the Ljungstrom preheater of Patent 1,652,025 dated December 6, 1927, mounted in the vertical or horizontal position. As a specific example of this process, the hot ue gas from the combustion of fuel oil containing 1.7 sulfur (expressed as elemental sulfur), the gas containing approximately 20 p.p.m. of sulfur trioxide, is fed to such an air preheater, mounted in the vertical position, at a temperature of 700 F. and therein heat-exchanged with ambient air having an inlet temperature of 70 F. The exit gas temperature from the air preheater was 200 F., well below the acid dew point temperature of the exit gas (approximately 285 E), the `CEMT being 135 F. It is to be noted that this process does not require the use of the conventional air heater for the ambient air.

A modification of the above described process involves the use of a multiple layer air preheater, designed for 135 F. CEMT exit conditions, with further provision for approximately 150 CEMT operation on the remaining layers.

In carrying out the further modification of the process of the present invention, utilizing the sulfur trioxide-removal steps of said copending application, any nely divided alkaline solid material can be used, including the alkaline earth metal oxides, hydroxides and carbonates, such as, for example, calcium carbonate, magnesium carbonate, limestone, dolomitic limestone, cement dust (a highly alkaline by-product from the manufacture f Portland cement), and the like. It is necessary to utilize a molar excess of the alkaline material over that theoretically required to react with the quantity of sulfur trioxide in the gas. It has been found that the neutralization reaction is virtually complete when two and one-half to three times the stoichiometric amount is added.

The expression inely divided as used herein is intended to include materials containing over 90% of particles smaller than 200 mesh, preferably smaller than 325 mesh. Superior results are obtained with finely divided alkaline materials composed of particles of progressively decreasing particle sizes over the range 325 mesh down to the order of one micron.

Referring now to the drawings, this modied and in fact preferred process of the present invention and the system used therein is illustrated in FIGURE 1 lof the drawings. In this system, the flue gas to be treated is passed through the line 5 to Ithe air preheater 6, wherein it is heat-exchanged with ambient air fed to the preheater through the line 7, the cooled flue gas, now below the :acid dew point temperature thereof, being fed through the line 10 to a baghouse generally indicated 11. A measured amount of the alkaline additive is injected into the gas stream by means of the feeder 12, which may comprise a screw type feeder `of conventional con-y struction. Y

The baghouse preferably comprises a housing 14 separated into two or more compartments by a partition 15, each compartment containing one or more tubular filter bags 16, 17 hung in the usual manner. Best results are obtained by using glass fiber bags, preferably treated with a silicone material, having a permeability in therange of 50 to 100. Fabrics which are subject to damage by the corrosive action of the gases should of course be avoided.

A common header 20 connects with the cleaned gas space on the outside of the bags in each compartment for removal of the cleaned gas from the baghouse and venting to the atmosphere. The flue gas is delivered to the interior of the bags 16 and 17 simultaneously through the lines 21 and 22, dampers 23 and 24 being open in normal operation. A purge fan 25 is provided, having inlet lines 28 and 29 leading from the lines 21 and 22 respectively, and an outlet line 30 leading to line During normal operation, the dampers 31 and 32 in lines 28 and 29 are closed. Thus, the flue gases pass from the line 10 through the lines 21 and 22 and then into the interior of the bags 16 and 17. The particulate matter in the flue gas, including the injected alkaline additive, is separated from the gas by impingement upon the fabric filter ysurface of the bags, the alkaline additive functioning as a iilter aid and building up a matrix through which the sulfur trioxide-laden gas must pass, bringing about the desired neutralization reaction for removal lof the sulfur trioxide. The cleaned gas emerges on the outside of the filter bags and is vented to atmosphere through the header 20.

After a period of such operation, the build-up of filter cake upon the interior surfaces of the bags 'becomes so great as to bring about an excessive pressure drop through the bags, requiring -cleaning of the bags and the baghouse compartments. This is preferably determined and accomplished automatically by suitable equipment (not shown) which includes instruments for continuously monitoring the pressure differential across the bags and for operating the cleaning cycle. Preferably, the cleaning cycle is initiated when the pressure differential reaches about three inches of water, whereupon the damper is closed and the damper 31 is opened, thus creating a negative pressure on the internal surfaces of the bag 16 and drawing a portion of the cleaned gas from the outlet header 20 in the reverse direction through the bag 16, permitting the bag to collapse and thereby dislodging the accumulated filter cake which falls into the ash removal hopper 40. The process is then repeated, that is the damper 23 is opened, the damper 31 is closed, the damper 24 is closed and the damper 32 is opened, collapsing the bag 17 and dislodging the lter cake into the hopper 41 in the same manner. In operation, the time between the cleaning cycles may vary from 1'0 minutes to as long as an hour, with the cleaning cycle itself requiring yonly a minute or two.

The following is a specific example of a pilot plant operation illustrating the process of the present invention, but it is to be understood that the invention is not to be limited to the specic details thereof:

A portion of the flue gas from a power plant burning -a residual fuel oil containing 1.7% sulfur (expressed as elemental sulfur) was treated in accordance with the process of the invention as follows: 1,450 c.f.m. of the flue gas, at a temperature of 300 F. and containing 210.4 ppm. of sulfur trioxide, was cooled by heat exchange with ambient air (70 F.) in the air preheater to a temperature of 223 F. (CEMT of 146 F.). To the cooled gas, having an acid dew point of 270 F. and containing 10.8 ppm. of sul-fur trioxide, was injected finely divided dolomitic limestone at a rate of approximately 1.5 pounds per hour, and the gas and entrained solids kwas passed through .the fbaghouse 11 having a total effective filtering area of 168 square feet. The cleaned gas efiiuent from the baghouse (1,310 c.f.m., 218 F.) had no visible plume, contained only 0.8 p.p.m. of sulfur trioxide and had an acid dew point of 115 F. The dolomitic limestone used in this example contained 37.5% magnesium carbonate, 62.1% calcium carbonate, small traces of silica, iron oxide and alumina, and had the following particle size distribution:

From the above description, it will be understood that .the process and the 'apparatus of the present invention provides an extremely efiicient and economical means for the removal of sulfur trioxide from exhaust gases containing the same, with the concomitant elimination of stack plume. the finely divided solid alkaline additives, which perform the dual functions of neutralization of the isulfur trioxide and aid in filtration. As a filtration aid, the additives function mechanically by permitting the particulate matter to accumulate as a soft porous filter cake in contrast to non-:additive filter cakes which are dense, highly hydroscopic and have a low permeability.

Regardless of Whether the process is used in conjunction with the sulfur trioxide-removal steps, it is lof great importance in improving efficiency of operation as in a steam generating plant. By this process, reductions in exit gas temperatures of 40 F. and more are easily obtained. Considering that a reduction in exit gas temperature of about 40 F. will produce a 1% increase in boiler efiiciency, and further considering that such an increase would amount to several hundred thousand dollars in savings in operation of such a plant over its life, it is believed that the importance of the present invention can be readily appreciated. Having fully described our invention, it is t-o be understood that we do not wish to be limited to the details set forth but our invention is of the full scope of the appended claims.

We claim:

1. In a process -for operation of an oil-fired steam generator of the type having an air preheater and utilizing a fuel comprising a sulfur-containing oil, the step cornprising passing the hot gaseous products of combustion containing sulfur trioxide through said air preheater in This is accomplished through the use of 6 heat-exchange relation with ambient air under conditions such that the CEMT is in the range of between about one hundred and seventy degrees Fahrenheit and one hundred and twenty degrees Fahrenheit and such that an aerosol of hydrated sulfur trioxide is formed which is passed through and out of said air preheater.

V2. The process of claim 1, wherein the CEMT is in the range of between about 155 and 120 F.

3. The process of claim 1, wherein the CEMT is in the range of l30 F.

4. The process of claim 1, including the steps of entraining a finely divided solid alkaline material into the cooled gases, and passing said gases into contact with a filter surface and building up a permeable filter cake of said material on said surface, whereby vsulfur trioxide is separated from said gases on said filte-r cake in the form of the sulfate salt of said material.

5. The process of claim 4, wherein the CEMT is in the range of between about and about 120 F.

6. The process of claim 5, wherein said material is composed of particles of progressively decreasing particle size over the range 325 mesh down to the order of one micron.

7. The process of claim 6, wherein said material comprises dolomitic limestone.

8. A heat exchange process comprising the steps of bringing ambient air into contact with hot gases of combustion containing sulfur trioxide in a heat exchanger such that said gases are cooled to a temperature corresponding to a CEMT in the range of between about one hundred seventy degrees Fahrenheit and one hundred and twenty degrees Fahrenheit, forming an aerosol Iof hydrated sulfur trioxide in said heat exchanger and exerting sufficient pressure on said air and said -gases to pass said aerosol through and out of said heat exchanger.

9. The process of claim 8, wherein the temperature corresponds to a CEMT in the range of between about 155 and about 120 F.

10. The process ofclaim 8, including the steps of entraining a finely divided solid alkaline material into the cooled gases, and passing -said gases into contact with a filter surface and building up a permeable filter cake of said material on said surface, whereby sulfur trioxide is separated from -said gases on said filter cake in the form of the sulfate salt of said material.

11. The process of claim 10, wherein the amount of said material is chemically equivalent to 21/2 to 3 mols of calcium carbonate per mol of sulfur trioxide contained K in said gas.

References Citedby the Examiner UNITED STATES PATENTS 2,718,453 9/ 1955 Beckman 23-2.1 2,919,174 12/1959 Pring 9.3-2.1 2,990,161 6/ 1961 Blaskowski 165-1 2,992,065 7/1961 Feustel et al. 23.-2.1

OSCAR R. VERTIZ, Primary Examiner.

MAURICE A. BRINDISI, Examiner.

E. C. THOMAS, Assistant Examiner. 

1. IN A PROCESS FOR OPERATION OF AN OIL-FIRED STEAM GENERATOR OF THE TYPE HAVING AN AIR PREHEATER AND UTILIZING A FUEL COMPRISING A SULFUR-CONTAINING OIL, THE STEP COMPRISING PASSING THE HOT GASEOUS PRODUCTS OF COMBUSTION CONTAINING SULFUR TRIOXIDE THROUGH SAID AIR PREHEATER IN HEAT-EXCHANGE RELATION WITH AMBIENT AIR UNDER CONDITIONS SUCH THAT THE CEMT IS IN THE RANGE OF BETWEEN ABOUT ONE HUNDRED AND SEVENTY DEGREES FAHRENHEIT AND ONE HUNDRED AND TWENTY DEGREES FAHRENHEIT AND SUCH THAT AN AEROSOL OF HYDRATED SULFUR TRIOXIDE IS FORMED WHICH IS PASSED THROUGH AND OUT OF SAID AIR PREHEATER.
 4. THE PROCESS OF CLAIM 1, INCLUDING THE STEPS OF ENTRAINING A FINELY DIVIDED SOLID ALKALINE MATERIAL INTO THE COOLED GASES, AND PASSING SAID GASES INTO CONTACT WITH A FILTER SURFACE AND BUILDING UP A PERMEABLE FILTER CAKE OF SAID MATERIAL ON SAID SURFACE, WHEREBY SULFUR TRIOXIDE IS SEPARATED FROM SAID GASES ON SAID FILTER CAKE IN THE FORM OF THE SULFATE SALT OF SAID MATERIAL. 